MessagesBuilding with Claude

Stop reasons and fallback

Learn what each stop_reason value means and how to handle truncation, tool use, paused turns, and refusals in your application.

Every Messages API response includes a stop_reason field that tells you why Claude stopped generating. Check this field to decide whether to use the response as-is, continue the conversation, retry, or fall back to another model.

For the full response schema, see the Messages API reference.

Quick reference

Value	When it occurs	What to do
`end_turn`	Claude finished its response naturally.	Use the response.
`max_tokens`	The response reached your `max_tokens` limit.	Raise `max_tokens` or continue the response.
`stop_sequence`	Claude emitted one of your `stop_sequences`.	Read `stop_sequence` to see which one fired.
`tool_use`	Claude is calling a tool.	Run the tool and return the result. A server tool call still missing its result block completes in a later response.
`pause_turn`	A server-tool loop reached its iteration limit.	Send the assistant content back to continue.
`refusal`	Claude declined to respond.	Read `stop_details` and retry on a fallback model.
`model_context_window_exceeded`	The response filled the model's context window.	Treat the response as truncated.

The stop_reason field

The stop_reason field is part of every successful Messages API response. Unlike errors, which indicate failures in processing your request, stop_reason tells you why Claude completed its response generation.

Example response

{
  "id": "msg_01234",
  "type": "message",
  "role": "assistant",
  "content": [
    {
      "type": "text",
      "text": "Here's the answer to your question..."
    }
  ],
  "stop_reason": "end_turn",
  "stop_sequence": null,
  "stop_details": null,
  "usage": {
    "input_tokens": 100,
    "output_tokens": 50
  }
}

Stop reason values

end_turn

The most common stop reason. Indicates Claude finished its response naturally.

client = anthropic.Anthropic()

response = client.messages.create(
    model="claude-opus-5",
    max_tokens=1024,
    messages=[{"role": "user", "content": "Hello!"}],
)
if response.stop_reason == "end_turn":
    # Process the complete response
    for block in response.content:
        if block.type == "text":
            print(block.text)

Sometimes Claude returns an empty response (exactly 2–3 tokens with no content) with stop_reason: "end_turn". This typically occurs when Claude interprets that the assistant turn is complete, particularly after tool results.

Common causes:

Adding text blocks immediately after tool results (Claude learns to expect the user to always insert text after tool results, so it ends its turn to follow the pattern)
Sending Claude's completed response back without adding anything (Claude already determined it's done, so it will remain done)

How to prevent empty responses:

# INCORRECT: Adding text immediately after tool_result
messages = [
    {"role": "user", "content": "Calculate the sum of 1234 and 5678"},
    {
        "role": "assistant",
        "content": [
            {
                "type": "tool_use",
                "id": "toolu_123",
                "name": "calculator",
                "input": {"operation": "add", "a": 1234, "b": 5678},
            }
        ],
    },
    {
        "role": "user",
        "content": [
            {"type": "tool_result", "tool_use_id": "toolu_123", "content": "6912"},
            {
                "type": "text",
                "text": "Here's the result",  # Don't add text after tool_result
            },
        ],
    },
]

# CORRECT: Send tool results directly without additional text
messages = [
    {"role": "user", "content": "Calculate the sum of 1234 and 5678"},
    {
        "role": "assistant",
        "content": [
            {
                "type": "tool_use",
                "id": "toolu_123",
                "name": "calculator",
                "input": {"operation": "add", "a": 1234, "b": 5678},
            }
        ],
    },
    {
        "role": "user",
        "content": [
            {"type": "tool_result", "tool_use_id": "toolu_123", "content": "6912"}
        ],
    },  # Just the tool_result, no additional text
]

If you still get empty responses after fixing the message structure, add a continuation prompt in a new user message rather than retrying with the empty response:

def handle_empty_response(client, messages):
    response = client.messages.create(
        model="claude-opus-5", max_tokens=1024, messages=messages
    )

    # Check if response is empty
    if response.stop_reason == "end_turn" and not response.content:
        # INCORRECT: Don't just retry with the empty response
        # This won't work because Claude already decided it's done

        # CORRECT: Add a continuation prompt in a NEW user message
        messages.append({"role": "user", "content": "Please continue"})

        response = client.messages.create(
            model="claude-opus-5", max_tokens=1024, messages=messages
        )

    return response

Best practices:

Never add text blocks immediately after tool results: This teaches Claude to expect user input after every tool use.
Don't retry empty responses without modification: Sending the empty response back won't help.
Use continuation prompts as a last resort: Only if these fixes don't resolve the issue.

max_tokens

Claude stopped because it reached the max_tokens limit specified in your request.

client = anthropic.Anthropic()
# Request with limited tokens
response = client.messages.create(
    model="claude-opus-5",
    max_tokens=10,
    messages=[{"role": "user", "content": "Explain quantum physics"}],
)

if response.stop_reason == "max_tokens":
    # Response was truncated
    print("Response was cut off at token limit")
    # Consider making another request to continue

If Claude's response is cut off because it hit the max_tokens limit, and the truncated response contains an incomplete tool use block, you'll need to retry the request with a higher max_tokens value to get the full tool use.

# Check if response was truncated during tool use
if response.stop_reason == "max_tokens":
    # Check if the last content block is an incomplete tool_use
    last_block = response.content[-1]
    if last_block.type == "tool_use":
        # Send the request with higher max_tokens
        response = client.messages.create(
            model="claude-opus-5",
            max_tokens=4096,  # Increased limit
            messages=messages,
            tools=tools,
        )

stop_sequence

Claude encountered one of your custom stop sequences.

client = anthropic.Anthropic()
response = client.messages.create(
    model="claude-opus-5",
    max_tokens=1024,
    stop_sequences=["END", "STOP"],
    messages=[{"role": "user", "content": "Generate text until you say END"}],
)

if response.stop_reason == "stop_sequence":
    print(f"Stopped at sequence: {response.stop_sequence}")

tool_use

Claude is calling a tool and expects you to run it.

client = anthropic.Anthropic()
weather_tool = {
    "name": "get_weather",
    "description": "Get the current weather in a given location",
    "input_schema": {
        "type": "object",
        "properties": {
            "location": {"type": "string", "description": "City and state"},
        },
        "required": ["location"],
    },
}


def execute_tool(name, tool_input):
    """Execute a tool and return the result."""
    return f"Weather in {tool_input.get('location', 'unknown')}: 72°F"


response = client.messages.create(
    model="claude-opus-5",
    max_tokens=1024,
    tools=[weather_tool],
    messages=[{"role": "user", "content": "What is the weather in San Francisco?"}],
)

if response.stop_reason == "tool_use":
    # Extract and execute the tool
    for block in response.content:
        if block.type == "tool_use":
            result = execute_tool(block.name, block.input)
            # Return result to Claude for final response

A tool_use response can also contain a server_tool_use block whose id has no matching result block. That server tool call is not finished, and this response does not carry its result. In the common case, Claude calls a server tool and one of your client tools in the same group of parallel tool calls: the API returns without running the server tool so that you can run the client tools first. There is no other marker for the state; detect it by checking each server_tool_use or mcp_tool_use block's id for a matching result block.

A mixed tool_use response

{
  "stop_reason": "tool_use",
  "content": [
    {
      "type": "server_tool_use",
      "id": "srvtoolu_01HxbWnMRmbWyMfUtJKC45rA",
      "name": "web_search",
      "input": { "query": "example article" }
    },
    {
      "type": "tool_use",
      "id": "toolu_01PjgRJLbXrXEMZwDNYLnBqk",
      "name": "run_command",
      "input": { "command": "uname -a" }
    }
  ]
}

The continuation is a user message of tool_result blocks, one for every tool_use block in the response (see Handle tool calls), with two extra rules: that message must contain nothing except the tool_result blocks, and the request must keep the same tools array. A resume request that no longer defines the waiting server tool fails with a 400 whose message ends but no `web_search` tool was provided. The API attaches your results to the still-open assistant turn, runs the deferred server tool (for paused code execution, resumes it), and continues the turn. For a server tool Claude called directly, the next response's content starts with the result block that answers the previous response's server_tool_use id.

The follow-up user message

{
  "role": "user",
  "content": [
    {
      "type": "tool_result",
      "tool_use_id": "toolu_01PjgRJLbXrXEMZwDNYLnBqk",
      "content": "Linux demo-host 6.8.0-52-generic x86_64 GNU/Linux"
    }
  ]
}

Adding anything after the tool_result blocks in that user message, such as text, ends the assistant turn; for a server tool Claude called directly, the request then fails with a 400 invalid_request_error that names the unresolved server tool:

`web_search` tool use with id `srvtoolu_01HxbWnMRmbWyMfUtJKC45rA` was found without a corresponding `web_search_tool_result` block

Leaving out a tool_result, or putting one after other content, fails earlier with the standard tool_use ids were found without tool_result blocks immediately after error instead. To give Claude more input, send it as a separate user message after the turn completes.

pause_turn

Returned when the server-side sampling loop reaches its iteration limit while executing server tools such as web search. The default limit is 10 iterations per request.

When this happens, the response may contain a server_tool_use block without a corresponding result block. To let Claude finish processing, continue the conversation by sending the response back as-is. A response that leaves a client tool_use block waiting on you never has a stop_reason of pause_turn: when Claude stops to call your tools, stop_reason is tool_use, and you continue it by sending the client tool_result blocks instead of the response itself.

response = client.messages.create(
    model="claude-opus-5",
    max_tokens=4096,
    tools=[{"type": "web_search_20250305", "name": "web_search"}],
    messages=[{"role": "user", "content": "Search for latest AI news"}],
)

if response.stop_reason == "pause_turn":
    # Continue the conversation by sending the response back
    messages = [
        {"role": "user", "content": "Search for latest AI news"},
        {"role": "assistant", "content": response.content},
    ]
    continuation = client.messages.create(
        model="claude-opus-5",
        max_tokens=4096,
        messages=messages,
        tools=[{"type": "web_search_20250305", "name": "web_search"}],
    )

refusal

Claude declined to generate a response. Safety classifiers return this stop reason as a normal HTTP 200 response, not an error.

client = anthropic.Anthropic()
response = client.messages.create(
    model="claude-opus-5",
    max_tokens=1024,
    messages=[{"role": "user", "content": "[Unsafe request]"}],
)

if response.stop_reason == "refusal":
    # Claude declined to respond
    print("Claude was unable to process this request")
    # Consider rephrasing or modifying the request

On a refusal, the stop_details object identifies the policy category that triggered it. The categories and the full refusal response shape are covered on Refusals and fallback. stop_details is null for all stop reasons other than refusal.

A refused request on Claude Fable 5.1, Claude Fable 5, or Claude Opus 5 can usually be served by retrying on another Claude model. Refusals and fallback shows how to set up that retry, server-side or in your client. If you build the retry yourself from Claude Fable 5.1, Claude Fable 5, or Claude Opus 5, fallback credit covers how to avoid paying the prompt-cache cost twice.

model_context_window_exceeded

Claude stopped because it reached the model's context window limit. This lets you request the maximum possible tokens without knowing the exact input size.

# Request with maximum tokens to get as much as possible
response = client.beta.messages.create(
    model="claude-opus-5",
    max_tokens=20000,  # Python SDK requires streaming for max_tokens above ~21k
    messages=[
        {"role": "user", "content": "Large input that uses most of context window..."}
    ],
)

if response.stop_reason == "model_context_window_exceeded":
    # Response hit context window limit before max_tokens
    print("Response reached model's context window limit")
    # The response is still valid but was limited by context window

Best practices for handling stop reasons

Always check stop_reason

Make it a habit to check the stop_reason in your response handling logic:

def handle_response(response):
    if response.stop_reason == "tool_use":
        return handle_tool_use(response)
    elif response.stop_reason == "max_tokens":
        return handle_truncation(response)
    elif response.stop_reason == "model_context_window_exceeded":
        return handle_context_limit(response)
    elif response.stop_reason == "pause_turn":
        return handle_pause(response)
    elif response.stop_reason == "refusal":
        return handle_refusal(response)
    else:
        # Handle end_turn and other cases
        return next(
            (block.text for block in response.content if block.type == "text"), ""
        )

Handle truncated responses gracefully

When a response is truncated because of token limits or the context window, append a notice so the reader knows the output is incomplete. To continue generating from where the response left off instead, see Ensuring complete responses.

def handle_truncated_response(response):
    text = next((block.text for block in response.content if block.type == "text"), "")
    if response.stop_reason in ["max_tokens", "model_context_window_exceeded"]:
        if response.stop_reason == "max_tokens":
            note = "[Response truncated due to max_tokens limit]"
        else:
            note = "[Response truncated due to context window limit]"
        return f"{text}\n\n{note}"
    return text

Implement retry logic for pause_turn

When using server tools, the API may return pause_turn if the server-side sampling loop reaches its iteration limit (default 10). Handle this by continuing the conversation:

def handle_server_tool_conversation(client, user_query, tools, max_continuations=5):
    """
    Handle server tool conversations that may require multiple continuations.

    The server runs a sampling loop when executing server tools. If the loop
    reaches its iteration limit, the API returns pause_turn. Continue the
    conversation by sending the response back to let Claude finish.
    """
    messages = [{"role": "user", "content": user_query}]

    for _ in range(max_continuations):
        response = client.messages.create(
            model="claude-opus-5", max_tokens=4096, messages=messages, tools=tools
        )

        if response.stop_reason != "pause_turn":
            # Claude finished processing - return the final response
            return response

        # pause_turn: replace the full message list to maintain alternating roles
        messages = [
            {"role": "user", "content": user_query},
            {"role": "assistant", "content": response.content},
        ]

    # Reached max continuations - return the last response
    return response

Stop reasons vs. errors

It's important to distinguish between stop_reason values and actual errors:

Stop reasons (successful responses)

Part of the response body
Indicate why generation stopped normally
Response contains valid content

Errors (failed requests)

HTTP status codes 4xx or 5xx
Indicate request processing failures
Response contains error details

client = anthropic.Anthropic()

try:
    response = client.messages.create(
        model="claude-opus-5",
        max_tokens=1024,
        messages=[{"role": "user", "content": "Hello!"}],
    )

    # Handle successful response with stop_reason
    if response.stop_reason == "max_tokens":
        print("Response was truncated")

except anthropic.APIStatusError as e:
    # Handle actual errors
    if e.status_code == 429:
        print("Rate limit exceeded")
    elif e.status_code == 500:
        print("Server error")

Streaming considerations

When using streaming, stop_reason is:

null in the initial message_start event
Provided in the message_delta event
Not provided in any other events

client = anthropic.Anthropic()

with client.messages.stream(
    model="claude-opus-5",
    max_tokens=1024,
    messages=[{"role": "user", "content": "Hello!"}],
) as stream:
    for event in stream:
        if event.type == "message_delta":
            stop_reason = event.delta.stop_reason
            if stop_reason:
                print(f"Stream ended with: {stop_reason}")

Common patterns

Handling tool use workflows

def complete_tool_workflow(client, user_query, tools):
    messages = [{"role": "user", "content": user_query}]

    while True:
        response = client.messages.create(
            model="claude-opus-5", max_tokens=1024, messages=messages, tools=tools
        )

        if response.stop_reason == "tool_use":
            # Execute tools and continue
            tool_results = execute_tools(response.content)
            messages.append({"role": "assistant", "content": response.content})
            messages.append({"role": "user", "content": tool_results})
        else:
            # Final response
            return response

Ensuring complete responses

def get_complete_response(client, prompt, max_attempts=3):
    messages = [{"role": "user", "content": prompt}]
    full_response = ""

    for _ in range(max_attempts):
        response = client.messages.create(
            model="claude-opus-5", messages=messages, max_tokens=4096
        )

        full_response += next(
            (block.text for block in response.content if block.type == "text"), ""
        )

        if response.stop_reason != "max_tokens":
            break

        # Continue from where it left off
        messages = [
            {"role": "user", "content": prompt},
            {"role": "assistant", "content": full_response},
            {"role": "user", "content": "Please continue from where you left off."},
        ]

    return full_response

Getting maximum tokens without knowing input size

With the model_context_window_exceeded stop reason, you can request the maximum possible tokens without calculating input size:

def get_max_possible_tokens(client, prompt):
    """
    Get as many tokens as possible within the model's context window
    without needing to calculate input token count
    """
    response = client.beta.messages.create(
        model="claude-opus-5",
        messages=[{"role": "user", "content": prompt}],
        max_tokens=20000,  # Python SDK requires streaming for max_tokens above ~21k
    )

    if response.stop_reason == "model_context_window_exceeded":
        # Got the maximum possible tokens given input size
        print(
            f"Generated {response.usage.output_tokens} tokens (context limit reached)"
        )
    elif response.stop_reason == "max_tokens":
        # Got exactly the requested tokens
        print(f"Generated {response.usage.output_tokens} tokens (max_tokens reached)")
    else:
        # Natural completion
        print(f"Generated {response.usage.output_tokens} tokens (natural completion)")

    return next((block.text for block in response.content if block.type == "text"), "")

Next steps

Refusals and fallback

Retry refused requests on a fallback model, server-side or in your client.

Tool Runner (SDK)

Let the SDK manage the tool_use loop, result formatting, and retries for you.

Streaming messages

Read stop_reason from the message_delta event when streaming.

Errors

Handle 4xx and 5xx HTTP errors, which are distinct from stop reasons.

Quick reference

The stop_reason field

Stop reason values

end_turn

Empty responses with end_turn

max_tokens

Incomplete tool use blocks

stop_sequence

tool_use

pause_turn

refusal

model_context_window_exceeded

Best practices for handling stop reasons

Always check stop_reason

Handle truncated responses gracefully

Implement retry logic for pause_turn

Stop reasons vs. errors

Stop reasons (successful responses)

Errors (failed requests)

Streaming considerations

Common patterns

Handling tool use workflows

Ensuring complete responses

Getting maximum tokens without knowing input size

Next steps